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November 08, 2011

Hundreds of exoplanets have been discovered but we could soon detect exomoons

Arxiv - The Detectability of Moons of Extra-Solar Planets (H/T Universe Today)

387 page pdf

The detectability of moons of extra-solar planets is investigated, focusing on the time-of-arrival perturbation technique, a method for detecting moons of pulsar planets, and the photometric transit timing technique, a method for detecting moons of transiting planets. Realistic thresholds are derived and analyzed in the in the context of the types of moons that are likely to form and be orbitally stable for the lifetime of the system.

For the case of the time-of-arrival perturbation technique, the analysis is conducted in two stages. First, a preliminary investigation is conducted assuming that planet and moon's orbit are circular and coplanar. This analysis is then applied to the case of the pulsar planet PSR B1620-26 b, and used to conclude that a stable moon orbiting this pulsar planet could be detected, if its mass was over 5% of its planet's mass (2.5 Jupiter masses), and if the planet-moon distance was ~ 2% of the planet-pulsar separation (23 AU). Time-of-arrival expressions are then derived for mutually inclined as well as non-circular orbits.

For the case of the photometric transit timing technique, a different approach is adopted. First, analytic expressions for the timing perturbation due to the moon are derived for the case where the orbit of the moon is circular and coplanar with that of the planet and where the planet's orbit is circular and aligned to the line-of-sight, circular and inclined with respect to the line-of-sight or eccentric and aligned to the line-of-sight. Second, the timing noise is investigated analytically, for the case of white photometric noise, and numerically, using SOHO lightcurves, for the case of realistic and filtered realistic photometric noise.




There are three mechanisms by which planets may obtain satellites. The simplest is for them to simply form together from a single accretion disk. Another is that a massive impact may knock material off of a planet which forms into a satellite as astronomers believe happened with our own Moon. Some estimates have indicated that such impacts should be frequent and as many as 1 in 12 Earth like planets may have formed moons in this way. Lastly, a satellite may be a captured asteroid or comet as is likely for many of the moons of Jupiter and Saturn.

Over the course of this thesis, the question of the detectability of extrasolar moons has been addressed in various ways. First, the pertinent issues were introduced by considering the types of moons likely to form and be retained around extra-solar planets (see chapter 3) and the suite of methods presented in the literature for detecting them (see chapter 4). Then, two of these methods, pulse time-of-arrival perturbation, for the case of moons of pulsar planets (see chapters 5 and 6) and photometric transit timing, for the case of moons of transiting planets (see chapters 7, 8, 9 and 10) were investigated in detail, in turn. As these two sections of work were quite independent, the results and future research directions for each will be discussed in turn.

Moon detection by pulse time-of-arrival perturbation

The detectability of moons of pulsar planets was rst considered using a simple model. In particular, it was assumed that the orbit of the planet and moon were both circular and in the same plane. This model was then applied to the case of the pulsar planet PSR B1620-26 b and used to exclude the existence of moons with mass greater than 0.125 Jupiter masses, a distance of greater than 0.46AU away from the planet. This was the first published limit on the mass and orbital characteristics of a moon of a pulsar planet and, to my knowledge, the third published limit on the moon of an extrasolar planet.
Moon detection by photometric transit timing

This thesis also addressed the detection of moons of transiting planets through an in-depth look at the photometric transit timing technique. A preliminary investigation was conducted using a Monte Carlo simulation with 500 realisations to see if moons could be detected. This was followed up with the work which presented a formula for the amplitude of this perturbation. In this context, the work presented in this thesis had ve main objectives, which will be discussed individually.

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